1. Field of the Invention
The present invention relates to a nondestructive test method for quantitatively determining fatigue of ferromagnetic construction materials, or of the structure comprised of such materials.
2. Description of the Related Art
Conventional nondestructive test methods for determining fatigue of materials are generally based on investigation of generation and growth of cracks in the material, and thus, it is highly important to find out as minute cracks as possible. With such a conventional nondestructive test method, it is practically impossible to evaluate metal fatigue of the material before cracks are generated.
There are also other types of nondestructive fatigue test methods known, which can be applied to ferromagnetic construction materials or structures comprised of such construction materials. One of such test methods is for measurement of the coercive force, and another method is for measurement of the magnetic susceptibility of the test material in the range approaching to saturation. It is known that the former method has less measurement sensitivity than the latter method, and such measurement sensitivity of the former method degrades when the materials that have more progressed metal fatigue are measured.
It is therefore a primary object of the present invention to provide an improved test method for nondestructively determining the metal fatigue of ferromagnetic construction materials, which advantageously eliminates the above-mentioned problems of the prior art.
One aspect of the present invention resides in a method for nondestructively determining metal fatigue of test ferromagnetic construction materials having a known, initial tensile stress "sgr"0, by quantifying a change in effective stress due to aging of the materials. The test method according to the present invention comprises the following three steps.
The first step is to measure the coercive force (Hc) and the magnetic susceptibility ("khgr"H) of a test material under a magnetic field having a coercive force(Hc).
The second step is to determine an effective tensile stress ("sgr") by putting said coercive force (Hc) and said magnetic susceptibility ("khgr"H) into a following first equation:
"sgr"=a(Hc/"khgr"H)nxe2x80x83xe2x80x83(1)
where a and n are known constants determined by the internal structure of the test material.
Finally, the third step is to determine a change in effective tensile stress of the test material, by comparing said effective tensile stress ("sgr") of the test material with the initial tensile stress ("sgr"0) of the test material.
Another aspect of the present invention resides in an apparatus for nondestructively determining metal fatigue of test ferromagnetic construction materials having a known, initial tensile stress ("sgr"0), by quantifying a change in the effective stress due to aging of the test materials. The apparatus according to the present invention comprises:
i) measuring means for measuring the magnetic susceptibility ("khgr"H) of a test material in its aged state, under a magnetic field having a coercive force (Hc);
ii) stress calculation means for calculating and thereby determining an effective tensile stress ("sgr") of the test material, by putting said coercive force (Hc) and said magnetic susceptibility ("khgr"H) into a following first equation:
"sgr"=a(Hc/"khgr"H)nxe2x80x83xe2x80x83(1)
where a and n are known constants determined by the internal structure of the test material; and
iii) evaluation means for determining a change in the effective stress of the test material due to aging thereof, by comparing the current tensile stress ("sgr") of the test material with its initial tensile stress ("sgr").
The nondestructive test apparatus according to the present invention, as a whole, may be comprised of a personal computer installed with programs based on the algorithm which enables execution of the above steps.
The principle of the present invention will be described below with reference to the experimental test data. To elucidate the interrelationship between the mechanical and magnetic properties of steel materials, test materials were prepared which consist of a pure iron single crystal, polycrystalline pure iron, and low-alloy steel A533B, respectively. These test materials were formed into samples having shapes as shown in FIGS. 1(a), 1(b) and 1(c), respectively, which are to be subjected to tensile and hysteresis loop tests. The samples as shown in FIG. 1(a) were used for the tensile test, while the samples as shown in FIG. 1(b) or 1(c) were used for the hysteresis loop test. As for the hysteresis loop test, the polycrystalline pure iron and low-alloy steel A533B took the shape of FIG. 1(b) while the pure iron single crystal took the shape of FIG. 1(c). Table 1 below shows the chemical composition of the low-alloy steel A533B submitted to the test.
FIGS. 2 to 4 illustrate the stress-strain characteristics of the test samples, obtained from the tensile test. FIG. 2 represents the results from Fe single crystal samples, and shows that the strain rate (i.e., extension rate) is 1.5%/min. FIG. 3 represents the results from Fe polycrystalline samples, and shows that the strain rate is 1.2%/min, and FIG. 4 represents the results from a low-alloy steel A533B sample, and shows that the strain rate is 1.2%/min.
FIGS. 5 and 6 illustrate the magnetization curves obtained from the hysteresis loop test after the application of stresses. FIG. 5 shows the hysteresis loop characteristics of Fe single crystal samples with plastic deformation of stresses (0 MPa, 55 MPa, or 115 MPa), while FIG. 6 shows the hysteresis loop characteristics of Fe polycrystalline samples with plastic deformation of stresses (0 MPa, 550 MPa, or 663 MPa). The applied stresses were chosen to be equal to 0 MPa and the stress that develops just before fracture, both of which had been obtained from a preparatory tensile test, and the above mentioned intermediate stresses had been chosen between these values for plastic deformation.
From the magnetization curve of test materials as depicted in FIGS. 5 and 6, it is possible to determine the coercive force Hc (the magnetic field intensity H at the flux density B=0) of the individual test material related to the tensile stress "sgr". FIG. 7 is obtained when the coercive force Hc is plotted against the tensile stress "sgr". The solid triangles (▴), solid circles (xe2x97xaf) and solid diamonds (♦) represent the results obtained from Fe single crystal material, Fe polycrystalline material, and low-alloy steel material, respectively.
Moreover, from the gradient of the magnetization curve of test materials near the flux density B=0 as depicted in FIGS. 5 and 6, it is possible to determine the magnetic susceptibility (H corresponds to the coercive force Hc). Thereby, FIG. 8 is obtained when a ratio of the coercive force Hc and the magnetic susceptibility "khgr"H at Hc, A=Hc/"khgr"H is calculated, and the logarithmic values of A are plotted in relation to corresponding logarithmic values of the tensile stress "sgr". The solid triangles (▴), solid circles (xe2x97xaf) and solid diamonds (♦) represent the results obtained from Fe single crystal material, Fe polycrystalline material, and low-alloy steel material, respectively.
From FIG. 8, the inventor investigated that the relation of the tensile stress "sgr" and the value A is expressed by the following equation:
log("sgr")=log(a)+nlog(A)xe2x80x83xe2x80x83(2),
where A=Hc/"khgr"H.
That is, the equation (2) can be expressed by the same form of the equation (1) as follows:
"sgr"=a(A)nxe2x80x83xe2x80x83(3)
where the constants a and n are determined from the crystal structure of test materials. It is supposed that the single crystal pure iron, polycrystalline pure iron, and low-alloy A533B steel submitted to the test each has the body-centered cubic (BCC) lattice structure, and contains iron atoms as main ingredient, thus, the characteristics obtained with respect to those materials can be represented by a relevant equation which is expressed by the equation (3).
Thus, if the tensile stress a is unknown, by calculating the ratio A and substituting this value A to the equation (3), the tensile stress "sgr" can be obtained. This tensile stress a becomes a parameter of the mechanical strength of materials.
Moreover, the ratio A can be obtained by measuring the hysteresis loop nondestructively using the magnetic yoke which is provided coils or the coils provided on the test materials.
Therefore, with the method according to the present invention, it is possible precisely to determine the current stress of test materials by obtaining the coercive force Hc and the magnetic susceptibility "khgr"H corresponding to said Hc, and calculating the effective tensile stress "sgr" of the test materials by putting the value A which is the ratio of the coercive force Hc and the magnetic susceptibility "khgr"H into the equation which includes the known constants a and n termined by the internal structure of the materials:
"sgr"=a(A)nxe2x80x83xe2x80x83(3),
and by comparing the current tensile stress a of the test material with its initial tensile stress "sgr"0.
It is to be noted that when construction materials are aged, i.e., exposed to a stress over a long period, lattice defects, such as dislocations develop; and the effective stress of the material increases. In this context, the increased effective stress of the test materials in their aged state is the current stress of the materials.
Moreover, in the conventional fatigue test method, the metal fatigue of test materials is evaluated by measuring the coercive force and obtaining the relation between the coercive force and the effective tensile stress, so that as shown in FIG. 7, the values of the coercive force only changes several tens times between the minimum and the maximum values of the tensile stress. On the other hand, the test method according to the present invention determines metal fatigue of test materials based on the relation between the effective stress a and the value A which is the ratio of the coercive force Hc and the magnetic susceptibility "khgr"H allowing the value A changes to be about 8000 times from 2.3xc3x9710xe2x88x926 to 1.8xc3x9710xe2x88x922 between the maximum and minimum of the tensile stress a, as seen from Table 2. Thus, as shown in FIG. 8, the range of the value for evaluation is expanded. This indicates that the method according to the present invention is more significantly sensitive to change in the tensile stress, which serves as a parameter for evaluating metal fatigue of test materials.
FIG. 9 shows the relation of tensile stress a and the dislocation density xcfx81 based on the experiment. In this experiment, the hysteresis characteristic test is carried out for the test pieces after loading the tensile stress as shown in FIGS. 5 and 6, and the dislocation density of the test pieces is measured via the observation by means of electron microscope about each tensile stress. In FIG. 9, solid triangles (▴), solid circles (xe2x97xaf) and solid diamonds (♦) represent the results obtained from Fe single crystal material, Fe polycrystalline material, and low-alloy steel material, respectively. From the experimental result, it is noted that there is a simple relation between tensile stress and the dislocation density. It is well-known that there is a certain relation between the dislocation density and metal fatigue. Therefore, from the above-mentioned experimental results, if the effective tensile stress is ascertained, it is possible to obtain the degree of metal fatigue from the tensile stress nondestructively.
Therefore, the method according to the present invention can be applied to the measurement of polycrystalline ferromagnetic construction materials and low-alloy steel materials. With this method, it is possible to examine the dislocation density and its distribution even before occurrence of cracks in the material nondestructively, and also to measure the degree of metal fatigue of the materials if the measurement is performed to fatigued ferromagnetic construction material.
In the nondestructive test method according to the present invention, the initial tensile stress "sgr"0 of test materials may be obtained from the following equation:
"sgr"0=F/Sxe2x80x83xe2x80x83(4)
where F represents a force applied to test ferromagnetic construction materials, and S the sectional area of materials normal to the direction of the force. In this instance, assuming that the external and/or internal forces applied to test materials are known, the initial tensile stress "sgr"0 can readily be derived from the equation (4).
Alternatively, the aforementioned initial tensile stress "sgr"0 of test materials may be obtained from the equation (4) in the same manner as is in the effective tensile stress "sgr". In this instance, even when the external and/or internal forces applied to test materials are unknown, the initial tensile stress "sgr"0 can readily be derived as is the case with the current tensile stress "sgr".
Still further, in the nondestructive test method according to the present invention, a U-shaped magnetic yoke may be used for measuring the coercive force Hc of test ferromagnetic construction materials. It is then possible to perform a nondestructive measurement on the test materials having a shape which does not readily permit a coil to be wound around them.
Moreover, in this invention, it is possible to construct the apparatus for nondestructively determining metal fatigue of test ferromagnetic construction materials by combining the means which perform aforementioned each step of the method according to this invention. To put it concretely, this apparatus may comprises:
i) measuring means for measuring the magnetic susceptibility ("khgr"H) of test material in its aged state, under a magnetic field having a coercive force (Hc);
ii) stress calculation means for calculating and thereby determining an effective tensile stress ("sgr") of the test material, by putting said coercive force Hc and said magnetic susceptibility ("khgr"H) into a following first equation:
"sgr"=a(Hc/"khgr"H)nxe2x80x83xe2x80x83(1)
where a and n are known constants determined by internal structure of the test material; and
iii) evaluation means for determining a change in effective stress of the test material due to aging thereof, by comparing the current tensile stress ("sgr") of the test material with its initial tensile stress ("sgr"0).